As working machines, various types of working machines are known, such as, for example, hydraulic shovels and wheel loaders and so on, and haulage vehicles such as dump trucks and so on. In order to reduce the costs of fuel, these vehicles are equipped with diesel engines, and diesel oil is used as the fuel. Although many users of working machines use normal diesel oil, sometimes some other fuel, such as kerosene or the like, is used in an unauthorized manner by being mixed in with the diesel oil. This is because kerosene and the like are cheaper in price than diesel oil.
However, since in recent years it is a strong requirement upon companies to address themselves to environmental problems, accordingly the makers of working machines and so on control their diesel engines at a high level of sophistication, and design the main components thereof, in order to reduce the negative impact upon the environment. This type of high level engine control assumes as a premise that normal diesel oil is being used as the fuel. Accordingly, if kerosene which includes a lower amount of oil as compared to diesel oil or an inferior fuel which includes impurities is used, then, while it is not possible to obtain the anticipated engine performance, also there is a possibility that damage may be caused to the fuel injection system or the like of the engine, and there is a possibility that the life of the engine may be decreased.
Thus, techniques have been proposed for detecting properties of the fuel. As a first such prior art technique, a method is known of discriminating whether or not normal diesel oil is being used, on the basis of the amount of sulfur oxide which is included in the exhaust gases of the engine (see Patent Document #1). And, as a second such prior art technique, a method is known of discriminating between diesel oil and kerosene on the basis of the difference in specific gravity between diesel oil and kerosene (see Patent Document #2).    Patent Document #1: Japanese Laid-Open Patent Publication 2004-219269;    Patent Document #2: Japanese Laid-Open Patent Publication Heisei 2-20146.